DE 18 05 639 discloses a heater, where a heating wire is wound around one or two parallel, elongated support plates in the form of insulating material plates, the heating wire being pressed with the aid of a press on both sides against the edges of the insulating material plates in such a way that deflections occur embracing the plate edges in clip or clamp-like manner. A disadvantage of this construction is that in the vicinity of said clip-like deflections and as a result of its engagement on the insulating material plates, the heating wire becomes too hot and starts to glow, which greatly decreases the service life. Relative to the longitudinal extension of the insulating material plates, the heating wire also does not have an adequate length in order to provide a high heating power.
Thus, DE 26 44 093 proposes winding the heating wire helically about a single support plate, the heating wire being uniformly corrugated over its entire length so as in this way to obtain a greater heating wire length. In order to ensure the necessary stability of the heating wire, also in this construction the heating wire comes in the vicinity of the plate edges into clip-like contact with the support plate, the contact length being further increased compared with the disclosure of DE 18 05 639, which also gives rise to the aforementioned disadvantages.
Thus, in the known heaters, where a higher heating power was required, e.g. in DE 25 35 478 and DE 195 05 930 it has been proposed to pass the tips of a substantially zig-zag-shaped heating wire through openings in two support plates parallel to one another. However, this gives rise to the disadvantage that at the reversal points or tips of the wire, as a result of the lower air flow rate at these points, an overheating and glowing of the wire can occur. In addition, in the case of such known heaters it is considered disadvantageous that the heating wire takes up all the space between the support plates and it is consequently impossible to place a temperature or thermal element centrally between said support plates. In addition, with such a construction and as a result of the aforementioned circumstances, it is difficult to provide a return conductor in the vicinity of the plates in the case of one-sided electrical terminals for the heater. A further disadvantage is the complicated and therefore relatively expensive manufacture of such heaters.
The problem of the invention is to provide an electric heater according to the preamble having an adequate heating wire length for generating and giving out the necessary heating power, the heating wire heating in a relative uniform manner over its entire length so as in this way to avoid points with excessive heating, so that the heater according to the invention has a long service life. The construction is intended to be particularly suitable for heaters through which there is a transverse flow.